The present invention relates to an apparatus for inspecting defects in a periodic pattern.
In conventional apparatus for inspecting defects in a periodic pattern, the pattern is visually examined with a microscope. Since, visual examination is inherently inaccurate, another well known inspection technique employs the combination of coherent light rays and a spatial filter. In this inspection technique, the optical information of the periodic pattern is separated into periodic pattern information and non-periodic pattern information by Fourier transformation and the non-periodic pattern information is examined by blocking the periodic pattern information with a spatial filter. This technique is disclosed in two articles in Proceedings of the IEEE, September 1969, pp. 1634 to 1639, "Inspection of Integrated Circuit Photomasks with Intensity Spatial Filters" by Watkins, and April 1972, pp. 447 to 448, "Intensity Spatial Filtering Applied to Defect Detection in Integrated Circuit Photomasks" by Axelrod. Also in U.S. patent application Ser. No. 130,370, filed Mar. 14, 1980, the above named inventor describes an improved apparatus using this inspection technique having on omni-directional spatial filter. A disadvantage of this technique is that the spatial filter and the examined pattern must be precisely aligned or oriented with each other.
All the above inspecting apparatus also have the disadvantage that it is impossible to distinguish between an enlarged hole defect and a reduced hole defect. For example, a mesh plate with a number of periodic square holes may have one kind of defect which is an enlarged hole as shown in FIG. 1A and another kind of defect which is a reduced hole as shown in FIG. 1B. The inspection apparatus detects defects by solving the formula D.sup.2 =(E-P).sup.2 where E is light information from the examined mesh plate pattern, P is light information from an ideal periodic pattern and D is light information representing defects. When the defect is an enlarged hole, E&gt;P and when the defect is a reduced hole, E&lt;P. Accordingly, the prior art inspection apparatus cannot distinguish between the enlarged hole of FIG. 1A and the reduced hole of FIG. 1B.
Sometimes it is necessary to distinguish between an enlarged hole and a reduced hole. For example, the mesh plate with a reduced hole shown in FIG. 1B can be repaired by manually removing the intruding part 10 to thereby improve the yield. Also, there is sometimes a difference in the acceptable tolerance between an enlarged hole and a reduced hole.